如何让Matplotlib Pyplot图例中的Hatches填充图案显示美观

问题原因 #

Matplotlib的hatch填充图案基于全局坐标空间绘制，而非图例中Patch的局部边界框，导致小尺寸Patch的hatch无法居中，出现对齐偏移。

解决方案 #

方法一：自定义居中Hatch的Patch类

重写Patch的get_hatch_transform方法，让hatch图案相对于Patch自身边界框居中：

import matplotlib.patches as mpatches
from matplotlib.transforms import TransformedBbox, Bbox
import matplotlib.pyplot as plt

# 自定义居中hatch的Patch类
class CenteredHatchPatch(mpatches.Patch):
    def __init__(self, hatch, **kwargs):
        super().__init__(**kwargs)
        self.hatch = hatch

    def get_hatch_transform(self):
        # 获取Patch的局部边界框
        bbox = TransformedBbox(Bbox.from_bounds(0, 0, self.get_width(), self.get_height()),
                               self.get_transform())
        # 调整变换矩阵，让hatch以Patch中心为原点
        hatch_trans = self.get_transform().inverted()
        hatch_trans = hatch_trans.translate(bbox.width/2, bbox.height/2)
        hatch_trans = hatch_trans.scale(1/bbox.width, 1/bbox.height)
        return hatch_trans

# 替换原图例元素中的Patch为自定义类
legend_elements=[
    CenteredHatchPatch(hatch="\\\\", facecolor='none', label='not unitary', edgecolor="#8D8896", lw=1),
    CenteredHatchPatch(hatch="//", facecolor='none', label='not bfb', edgecolor="#752692", lw=1),
    CenteredHatchPatch(hatch="---", facecolor='none', label='STU obs.', edgecolor="#124C8A", lw=1),
    CenteredHatchPatch(hatch="xx", facecolor='none', label='Vacuum stab.', edgecolor="#B9314F", lw=1),
    CenteredHatchPatch(hatch="||", facecolor='none', label='HB excl.', edgecolor="#F57200", lw=1),
    CenteredHatchPatch(hatch="++", facecolor='none', label='HS excl.', edgecolor="tab:green", lw=1),
    CenteredHatchPatch(hatch="//", facecolor='none', label='$h_{95}$ excl.', edgecolor="#4EA5D9", lw=1),
    mpatches.Patch(facecolor="darkgreen", label='stable')
]

fig, ax = plt.subplots()
ax.legend(handles=legend_elements, ncols=2, loc="lower center", bbox_to_anchor=(0.5, 1.05))
plt.show()

方法二：调整图例Patch尺寸

通过增大图例中Patch的显示尺寸，弱化hatch偏移的视觉效果，操作简单：

# 在legend调用中添加handlelength和handleheight参数
ax.legend(handles=legend_elements, ncols=2, loc="lower center", bbox_to_anchor=(0.5, 1.05),
          handlelength=2, handleheight=1.5)

方法三：用Line2D模拟Hatch图案

对于简单图案，直接用Line2D的marker或线条模拟，完全控制图案位置：

from matplotlib.lines import Line2D

# 重新定义图例元素，用Line2D替代部分Patch
legend_elements=[
    mpatches.Patch(facecolor='none', hatch="\\\\", label='not unitary', edgecolor="#8D8896", lw=1),
    # 用"+" marker模拟"++" hatch
    Line2D([], [], color='tab:green', lw=1, marker='+', linestyle='', markersize=8, label='HS excl.'),
    # 用水平线模拟"---" hatch
    Line2D([], [], color="#124C8A", lw=1, linestyle='-', label='STU obs.'),
    # 用叉号marker模拟"xx" hatch
    Line2D([], [], color="#B9314F", lw=1, marker='x', linestyle='', markersize=8, label='Vacuum stab.'),
    # 其他元素按需替换
    mpatches.Patch(facecolor="darkgreen", label='stable')
]

方法对比 #

• 自定义Patch：彻底解决居中问题，适配所有hatch类型，代码稍复杂。
• 调整尺寸：快速改善视觉效果，无需修改元素定义，但无法完全消除偏移。
• Line2D模拟：灵活性高，适合简单图案，需逐个调整元素样式。

内容的提问来源于stack exchange，提问作者TheMaker845